1. Field of the Disclosure
The present disclosure relates to a single mode (SM) fiber laser system configured with a multimode active fiber which is structured so that high-order modes propagating along the multimode fiber are substantially eliminated.
2. Prior Art Discussion
The use of multimode (MM) active fibers, i.e., fibers provided with a gain medium doped with rare-earth elements, permits rapid power scaling. However, increased powers are achieved at expense of some power loss in a single mode (SM) fiber laser system configured with a MM active fiber, or the poor quality of beam (high M2 factor1) in multimode laser systems. Both of the above are a result of excitation and amplification of high order transverse modes (HOM) which, on one hand, drain power from the pump light, thereby depriving a fundamental mode of some of this light power, and, on the other hand, couple with a fundamental mode. 1 A parameter for quantifying the output laser beam quality.
Many applications in need for high power SM output beams cannot tolerate even an insignificant loss of power. Similarly, MM laser systems with a high M2 factor are not acceptable for a variety of applications. Hence numerous techniques for filtering HOMs have been developed and are briefly discussed below.
One of the techniques for suppressing HOMs in a MM fiber without substantially affecting a single, fundamental mode provides for suitably designing the fiber index and dopant profiles. Still other techniques include configuring special cavities, tapering the fiber ends, or carefully adjusting the launch conditions of an input beam. Yet a further technique is associated with introducing bend losses, which can act as a form of distributed spatial filtering and suppress all but the fundamental mode of a MM fiber, yielding a single mode, powerful output. In some instances, the known methods produce satisfactory results but are cost-ineffective, in others, the known methods are simply do not yield the desired results.
A need, therefore, exists for a method of substantially eliminating high-order modes propagating along a multimode active fiber in a simple, cost-effective manner.
Another need exists for a MM fiber configured in accordance with the disclosed method.
Still a further need exists for a laser system configured with the disclosed MM fiber.